Highly efficient green electroluminescence of iridium(iii) complexes based on (1: H -pyrazol-5-yl)pyridine derivatives ancillary ligands with low efficiency roll-off

Article abstract of DOI:10.1039/c8tc01182f

Four iridium(iii) complexes, namely Ir-me, Ir-cf3, Ir-py, and Ir-ph, were synthesized, in which 2-(4-trifluoromethyl)phenylpyridine (tfmppy) was used as the main ligand and 2-(3-methyl-1H-pyrazol-5-yl)pyridine (mepzpy), 2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridine (cf3pzpy), 2,2′-(1H-pyrazole-3,5-diyl)dipyridine (pypzpy), and 2-(3-phenyl-1H-pyrazol-5-yl)pyridine (phpzpy) were applied as ancillary ligands, respectively. All complexes showed similar green light peaking at 494-499 nm with high phosphorescence quantum efficiency (0.76-0.82). The organic light-emitting diodes (OLEDs) with the structure of ITO/HATCN (hexaazatriphenylenehexacabonitrile) (5 nm)/TAPC (bis[4-(N,N-ditolylamino)-phenyl]cyclohexane, 50 nm)/Ir complexes (8 wt%): TCTA (4,4′,4′′-tri(9-carbazoyl)triphenylamine, 20 nm)/TmPyPB (1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene, 40 nm)/LiF (1 nm)/Al (100 nm) displayed high current efficiency with low efficiency roll-off. Moreover, the device based on the Ir-me complex exhibited the best performances with a maximum luminance of 38 155 cd m^-2, maximum current efficiency of 92 cd A^-1, and a maximum external quantum efficiency of 28.90%. These results suggested that green Ir(iii) complexes were obtained by modification of the ppy ligand and rational introduction of (1H-pyrazol-5-yl)pyridine derivatives as the ancillary ligands for high efficient OLEDs.

Full text of DOI:10.1039/c8tc01182f

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Nguyên T. K. Thanh, Xiaodi Su et al.
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High efficient green electroluminescence of iridium(III) complexes
based on (1H-pyrazol-5-yl)pyridine derivatives ancillary ligands
with low efficiency roll-off
Received 00th January 20xx,
Accepted 00th January 20xx
Ning Su1, GuangꢀZhao Lu1, YouꢀXuan Zheng1,2
*
DOI: 10.1039/x0xx00000x
Four iridium(III) complexes namely Ir-me, Ir-cf3, Ir-py and Ir-ph were synthesized, in which 2ꢀ(4ꢀ
trifluoromethyl)phenylpyridine (tfmppy) was used as main ligand and 2ꢀ(3ꢀmethylꢀ1Hꢀpyrazolꢀ5ꢀyl)pyridine
(mepzpy), 2ꢀ(3ꢀ(trifluoromethyl)ꢀ1Hꢀpyrazolꢀ5ꢀyl)pyridine (cf3pzpy), 2,2'ꢀ(1Hꢀpyrazoleꢀ3,5ꢀdiyl)dipyridine
(pypzpy) and 2ꢀ(3ꢀphenylꢀ1Hꢀpyrazolꢀ5ꢀyl)pyridine (phpzpy) were applied as the ancillary ligands,
respectively. All complexes showed similar green light peaked at 494ꢀ499 nm with high phosphorescence
quantum efficiency (0.76ꢀ0.82). The organic lightꢀemitting diodes (OLEDs) with the structure of ITO/HATCN
(hexaazatriphenylenehexacabonitrile) (5 nm)/ TAPC (bis[4ꢀ(N,Nꢀditolylamino)ꢀphenyl]cyclohexane, 50
nm)/Ir complexes (8 wt %): TCTA (4,4’,4’’ꢀtri(9ꢀcarbazoyl)triphenylamine, 20 nm)/ TmPyPB (1,3,5ꢀtri[(3ꢀ
pyridyl)ꢀphenꢀ3ꢀyl]benzene, 40 nm)/LiF (1 nm)/Al (100 nm) displayed high current efficiency with low
efficiency rollꢀoff. Moreover, the device based on Ir-me complex exhibited the best performances with a
maximum luminance of 38155 cd mꢀ2, the maximum current efficiency of 92 cd Aꢀ1, a maximum external
quantum efficiency of 28.90%. These results suggested that green Ir(III) complexes were obtained by
modification of the ppy ligand and rational introduction of (1Hꢀpyrazolꢀ5ꢀyl)pyridine derivatives as the
ancillary ligands for high efficient OLEDs.
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nonradioactive quenching processes increase will cause a serious
efficiency rollꢀoff.36ꢀ39 From the former publications, the nitrogen
heterocycle would enhance the electron affinity and the electron
Introduction
Cyclometalated iridium(III) complexes have received a great deal of
attention as phosphorescent materials in organic lightꢀemitting
diodes (OLEDs) in the past two decades due to their high
luminescent efficiency, good chemical stability and tunable emission
colour in the visible region.1ꢀ14 Among these Ir(III) complexes, green
phosphorescent emitters successfully draw the researchers’ attention
owing to its high current efficiency with low efficiency roll off even
at high current density.15ꢀ22 To tune the emission colour, the energy
gap is an important factor that determines the emission colour of
Ir(III) complexes.23ꢀ30 For example, the auxiliary ligands 2ꢀ
pyridinepyrazol or 2ꢀpyridinetriazole easily extended to the gap
between highest occupied molecular orbital (HOMO) and/or lowest
unoccupied molecular orbital (LUMO) of the Ir(III) complex, thus it
is believed that they would have an effect on the emission colour
tuning of the Ir(III) complex.31ꢀ35 Furthermore, it is wellꢀknown that
the balance of the electronꢀhole injection and transport is necessary
for highꢀefficiency OLEDs using Ir(III) complexes as emitters
because both the charge carrier balance deterioration and
mobility of the Ir(III) complexes.40ꢀ42
With this consideration, in this work, four Ir(III) complexes
namely Ir-me, Ir-cf3, Ir-py and Ir-ph were successfully
synthesized, respectively (Scheme 1), in which 2ꢀ(4ꢀ
trifluoromethyl)phenyl pyridine (tfmppy) was used as main ligand,
and
2ꢀ(3ꢀmethylꢀ1Hꢀpyrazolꢀ5ꢀyl)pyridine
(mepzpy),
2ꢀ(3ꢀ
(trifluoromethyl)ꢀ1Hꢀpyrazolꢀ5ꢀyl)pyridine (cf3pzpy), 2,2'ꢀ(1Hꢀ
pyrazoleꢀ3,5ꢀdiyl) dipyridine (pypzpy) and 2ꢀ(3ꢀphenylꢀ1Hꢀpyrazolꢀ
5ꢀyl)pyridine (phpzpy) were applied as the ancillary ligands. The
introduction of trifluoromethyl group in ppy ligand is beneficial to
increase steric hindrance of Ir(III) complexes and suppress tripletꢀ
triplet annihilation (TTA). It is found that the Ir(III) complexes with
different ancillary ligands displayed similar photophysical, thermal,
electrochemical and electroluminescence (EL) properties. It is worth
mentioning that the OLEDs employing the Ir(III) dopants exhibit
high current efficiency with low efficiency rollꢀoff. Especially, the
device based on Ir-me complex exhibited the best performances
with the maximum luminance (Lmax) of 38 155 cd mꢀ2, the maximum
current efficiency (ηc,max) of 92 cd Aꢀ1, the maximum external
quantum efficiency (EQEmax) of 28.90% and the maximum power
efficiency (ηp,max) of 67.53 lm Wꢀ1.
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R= CH3, mepzpy
R= CF3, cf3pzpy
O
NH
CF3
N
of Ir-cf3 complex also display the similar law. This phenomenon
confirms the sigma donation of the carbons (trans effect) in the Ir(III)
complexes.46ꢀ49 Moreover, from the single crystal packing (Fig. S2),
we can see clearly that the steric CF3 unit in the main ligand of Ir-
me complex have a headꢀtoꢀhead stacking mode, thus, there is a
weak πꢀπ stacking between adjacent Ir(III) complex, this is
beneficial to increase steric hindrance of Ir(III) complexes and
further suppress TTA effect, which is in good agreement with our
design original intention.
N
N
N2H4
EtOH
O
N
NaH
THF
O
R
C
O
O
+
R
R= pyridine, pypzpy
R= phenyl, phpzpy
R
CF3
N
Cl
Cl
2-ethoxyethonal
Pd(pph3)4
Ir
Br
CF3
Ir
+
H2
O
K2CO3
N
2
N
N
2
B(OH)2
tfmppy
CF3
F3
C
CF3
CF3
NH
N
N
N
Cl
2-ethoxyethonal
K2CO3
The thermal stabilities of the Ir(III) complexes were important for
the OLEDs fabrication and application. Therefore, the TGA
(thermogravimetric analysis) measurements (Fig. 2) were tested for
these complexes. The high decomposition temperatures (Td) above
370 oC are observed for all the Ir(III) complexes (Ir-me: 378 oC, Ir-
Ir
N
+
N
Ir
R
Ir
2
N
Cl
N
2
N
N
R
CF3
R= CH3, Ir-me
R= CF3, Ir-cf3
R= pyridine, Ir-py R= phenyl, Ir-ph
o
o
cf3: 383 oC, Ir-py: 412 C and Ir-ph: 400 C) with 5% loss of
Scheme 1 Synthetic route of Ir(III) complexes.
weight, which is beneficial for the OLEDs application.
Results and discussion
Synthesis and Characterization
100
Ir-me
Ir-cf3
Ir-py
Ir-ph
90
All the synthesis experiments were carried out under nitrogen and
solvents were dried prior to use. The main ligand tfmppy was
synthesized by Suzuki coupling reaction with good yield. The ꢁꢀ
chloroꢀbridged dimer [(tfmppy)2Ir(ꢁꢀCl)]2 was synthesized
according to the reported methods by heating IrCl3·3H2O (1 equiv)
and cyclometalated ligand tfmppy (2.5 equiv) in 2ꢀethoxyethanol
and water (v/v = 3/1) at 110 °C. The ancillary ligands (mepzpy,
cf3pzpy, pypzpy and phpzpy) were obtained by a successive
condensation and cyclization reactions.43 The Ir(III) complexes (Ir-
me, Ir-cf3, Ir-py and Ir-ph) were synthesized according to our
previous reports.36, 44, 45 The Ir(III) complexes were confirmed with
1H NMR, high resolution mass spectrometer (HRMS) and Xꢀray
crystallography (see ESI).
80
70
60
50
40
30
20
10
0
100
200
300
400
500
600
700
800
Temperature (oC)
Fig. 2 TGA curves of Ir(III) complexes.
Photophysical property
emission
absorption
Ir-me
Ir-me
Ir-cf3
Ir-py
Ir-ph
Ir-cf3
Ir-py
Ir-ph
Fig. 1 ORTEP diagram of Ir-me (CCDC No. 1819964) and Ir-cf3 complexes
(CCDC No. 1821899) with the atomꢀnumbering schemes. Hydrogen atoms
are omitted for clarity. Ellipsoids are drawn at the 50% probability level.
Single crystals of Ir-me and Ir-cf3 complexes were obtained from
vacuum sublimation and analysized by Xꢀray diffraction (Fig. 1).
The selected bond lengths were collected in the Table S1 and Table
S2 (ESI), respectively. The iridium atom of both complexes adopts a
distorted octahedral coordination geometry with two C^N
cyclometalated ligands and one N^N ligand. The bond lengths of Ir1ꢀ
N3 (2.144(11) Å) and Ir1ꢀN4 (2.103(9) Å) of Ir-me complex are
longer than that of Ir1ꢀN1 (2.043(10) Å) and Ir1ꢀN2 (2.052(12) Å),
respectively. Furthermore, the bond lengths of Ir01ꢀN3(N4, N1, N2)
300
400
500
600
Wavelength (nm)
Fig. 3 Normalized absorption and emission spectra of the iridium complexes
in dilute DCM (10ꢀ5 M) at RT.
The UV/vis absorption and photoluminescence (PL) spectra of the
Ir(III) complexes in dilute DCM solution (10ꢀ5 M) at room
temperature are shown in Fig. 3 and the relevant photophysical data
are summarized in Table 1. Similar absorption spectra are displayed
in the range of 260ꢀ500 nm and the highꢀenergy absorption bands
2 | J. Name., 2012, 00, 1-3
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below 350 nm are definitely ascribed to πꢀπ* transitions of the main show similar absorption and emission spectra indicating that the
ligand. However, the lowerꢀenergy absorption bands (350ꢀ500 nm) different ancillary ligand has little effect on their photophysical
1
3
can be assigned to the mixed MLCT and MLCT (metalꢀtoꢀligand properties.
chargeꢀtransfer) states or LLCT (ligandꢀtoꢀligand charge transfer)
transition through strong spinꢀorbit coupling of metal iridium atom.
As well known that the phosphorescence excited lifetime (τ) of
the Ir(III) complex is a crucial factor for determining the rate of TTA
The Ir(III) complexes showed similar green light emission peaked in the OLEDs. The longer phosphorescence τ would cause more
at 499 nm. While the Ir-cf3 complex showed a little blueꢀshifted serious TTA effect. In this case, the lifetimes of the complexes
emission peak at 494 nm with a shoulder at 523 nm. It is due to the measured in degassed DCM at RT are in the range of 1.86 ꢀ 2.34 ꢁs
withꢀdrawing ꢀCF3 group attached in the ancillary ligands. (Table 1), which can illustrate the phosphorescent origin of the
Obviously, at 77 K the two major emission peaks for all complexes excited states. Moreover, all the complexes show high
become sharper and the third shoulder peak also appears (ESI, Fig. phosphorescence quantum efficiency (0.76ꢀ0.82) and Ir-me displays
3
S1), which is attributed to the hybrid states such as major MLCT the highest yield of 0.82. These results suggest these materials are
3
and minor ILCT characters.36, 50, 51 In general, the Ir(III) complexes potential emitters for efficient OLEDs.
Table 1. Photophysical data of the Ir(III) complexes
PL(nm)b
77 K
Td
UVa
τd
EHOMO
ELUMO
c
Complexes
ΦPL
(oC)
(nm)
(ꢁs)
(eV)e
(eV)f
RT
Ir-me
Ir-cf3
Ir-py
Ir-ph
378
383
412
400
276,390,450
267,385,449
265,387,450
499,523
494,523
498,526
499,528
500,538,581
494,532,575
495,534,575
499,535,576
0.82
0.78
0.76
0.79
1.92
2.34
1.86
1.93
ꢀ5.55
ꢀ5.78
ꢀ5.62
ꢀ5.50
ꢀ3.07
ꢀ3.30
ꢀ3.14
ꢀ3.02
269,390,450
b
c
aMeasured in dilute DCM (10ꢀ5 M) at RT. Measured in dilute DCM (10ꢀ5 M) at RT or 77K. Measured in dilute DCM (10ꢀ5 M) in N2 atmosphere
according to the equation of Φs=Φr(ɳs2ArIs/ɳr2AsIr). dMeasured in dilute DCM (10ꢀ5 M) in N2 atmosphere. eCalculated from empirical equation: EHOMO
= ꢀ(Eox+ 4.8) eV. fCalculated from ELUMO = Egopt + EHOMO.
ancillary ligands. Moreover, the ancillary ligand of Ir-ph complex
owns the largest HOMO energy proportion of 76.45%, while with
respect to Ir-cf3 complex, its HOMO energy is almost no
Electrochemical property and Theoretical
Calculations
distribution in the ancillary ligand, but mainly located in tfmppy
ligand and iridium atom with almost the same ratio. In general, the
Ir(III) complexes display the similar LUMO energy distribution but
The HOMO and LUMO energy levels of the Ir(III) complexes
are important for the design of the OLEDs device structure. To
calculate these values, cyclic voltammetry were carried out using
a
different HOMO energy proportion owing to the distinct
ferrocene as the internal standard (Fig. S3). The HOMO energy
levels (EHOMO) can be calculated according to the equation of EHOMO
= ꢀ(Eox+ 4.8) eV. Based on the corresponding optical energy gaps
(Egopt) and EHOMO levels, the LUMO energy levels (ELUMO) are
estimated. The corresponding data are collected in Table 1 and Table
S3. Moreover, Ir-cf3 owns the lowest HOMO energy levels among
these complexes indicating that the withꢀdrawing CF3 unit in the
ancillary ligand has a great influence on their electrochemical
properties.
substituents attached in the ancillary ligand.
Moreover, density functional theory (DFT) calculations
employing Gaussian09 software with the B3LYP functional were
carried out to further investigate the electronic states and orbital
distributions of the Ir(III) complexes. The optimized HOMO/LUMO
structures of the Ir(III) complexes (Ir-me, Ir-cf3, Ir-py and Ir-ph)
are clearly shown in Fig. 4. And the corresponding HOMO and
LUMO electron cloud density distribution of each fragment of the
Ir(III) complexes were shown in Table S4. Obviously, the LUMO
energy distribution of the Ir(III) complexes were mainly located in
the main ligand (tfmppy) with a large proportion (as high as 94%),
while the HOMO energy distribution of Ir-me, Ir-py and Ir-ph
complexes were mainly distributed in metal iridium atom and the
Fig. 4 Theoretical (black) and experimental (red, determined by cyclic
voltammetry) HOMO/LUMO energy levels of the iridium complexes.
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Journal Name
Electroluminescent property
(hexaazatriphenylenehexacabonitrile,
5
nm)/TAPC (bis[4ꢀ(N,Nꢀ
ditolylamino)ꢀphenyl]cyclohexane, 50 nm)/Ir(III) complexes (8
wt%): TCTA (4,4’,4’’ꢀtri(9ꢀcarbazoyl)triphenylamine, 20 nm)/
TmPyPB (1,3,5ꢀtri[(3ꢀpyridyl)ꢀphenꢀ3ꢀyl]benzene, 40 nm)/ LiF (1
nm)/Al (100 nm) were fabricated (Scheme 2). For convenience, the
devices using Ir-me, Ir-cf3, Ir-py and Ir-ph emitters as dopants are
namely G1, G2, G3 and G4, respectively. HATCN and TAPC were
used as holeꢀinjection layer and holeꢀtransporting layer, respectively,
and TCTA was applied as host material. While TmPyPB and LiF
were utilized as electronꢀtransporting and electronꢀinjection layer,
respectively. Fig. 5 displays the EL spectra, luminance (L) versus
voltage (V), current efficiency (ηc) versus luminance, and power
efficiency (ηp) versus luminance characteristics of the OLEDs. In
addition, Table 2 collects the key EL data of G1ꢀG4 devices.
NC
CN
N
N
-1.8 eV
-2.4 eV
N
N
N
N
-2.7 eV
NC
CN
N
N
NC
CN
TAPC
HATCN
-4.3 eV
LiF/Al
N
-5.1 eV
ITO
-5.5 eV
N
N
-5.5 eV
N
-5.7 eV
-6.7 eV
N
N
N
TmPyPB
TCTA
-9.5 eV
Scheme 2. The OLEDs device structure and the molecular structures used in
this work.
In order to investigate the EL performances of these Ir(III)
complexes, the devices with the structure of ITO/HATCN
105
(a)
G1
(b)
G1
G2
G3
G4
G2
G3
G4
104
103
102
101
100
400
500
600
700
3
4
5
6
7
8
9
10
11
Wavelength (nm)
Voltage (V)
100
80
60
40
20
80
60
40
20
(c)
(d)
G1
G2
G3
G4
G1
G2
G3
G4
0
1000
2000
3000
4000
5000
0
1000
2000
3000
4000
5000
Luminance (cd m-2)
Luminance (cd m-2)
Fig. 5 Characteristics of devices with configuration ITO/HATCN (5 nm)/TAPC (50 nm)/Ir(III) complexes: TCTA (20 nm, 8 wt %)/ TmPyPB (40 nm)/LiF (1
nm)/Al (100 nm): (a) EL spectra; (b) LꢀV curves; (c) cꢀL curves; (d) pꢀL curves for G1ꢀG4
η
η
From Fig. 5a it can be observed almost identified EL spectra are devices with the Commission Internationale de 1’Eclairage (CIE)
observed for device G1, G3 and G4 peaked at 499 nm with a color coordinates at (0.27, 0.51), (0.21, 0.57), (0.23, 0.62) and (0.23,
shoulder at 526 nm at the dopant concentrations of 8 wt%, which 0.62) all display pure green OLEDs emissions, respectively.
indicates that the EL emission derives from the triplet excited states
All devices exhibited high efficiency with low efficiency rollꢀoff
of the Ir(III) complexes. While device G2 based on Ir-cf3 complex though at 10000 cd mꢀ2. From Fig. 5bꢀ5d it can be observed that the
displays a little blueꢀshifted EL emission, which is owing to withꢀ G1 device doping with Ir-me complex (ΦPL = 0.82) showed the
drawing ꢀCF3 unit of the ancillary ligand. Moreover, the intensities highest performances. Because four complexes have similar
of the EL shoulder peak at 526 nm are obviously different from the molecular structures and the variation of ancillary ligands have little
PL emissions in DCM at RT, which is owing to the different effects on their emissions, their EL performances mainly determined
luminescence mechanism. Furthermore, It is shown that G1ꢀG4 by their photoluminescence efficiencies. Respectively, the device G1
4 | J. Name., 2012, 00, 1-3
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displays the highest maximum luminance (Lmax) of 38155 cd mꢀ2, the cd Aꢀ1, 26.01% and 57.07 lm Wꢀ1. Due to the relative lowest ΦPL of
highest efficiencies with the maximum current efficiency (ηc,max) of Ir-py (ΦPL = 0.76), the device G3 displays the relative lowest
92 cd Aꢀ1, the maximum external quantum efficiency (EQEmax) of performances with Lmax of 35927 cd mꢀ2, ηc,max of 85.93 cd Aꢀ1,
28.90% and the maximum power efficiency (ηp,max) of 67.53 lm Wꢀ1. EQEmax of 26.61% and ηp,max of 69.19 lm Wꢀ1. Because the Ir-ph
Furthermore, the device also shows low efficiency rollꢀoff. Even at emitter has the high ΦPL (0.79) only lower than that of Ir-me, the
the brightness of 1000 cd mꢀ2, these efficiency values still keep at device G4 also has the second highest performances among four
79.97 cd Aꢀ1, 25.16% and 54.59 lm Wꢀ1. The device G2, which using devices with Lmax of 36067 cd mꢀ2, ηc,max of 90.29 cd Aꢀ1, EQEmax of
Ir-cf3 emitter (ΦPL = 0.78), exhibits the Lmax of 35541 cd mꢀ2 with 27.97% and ηp,max of 65.93 lm Wꢀ1. At the luminance of 1000 cd mꢀ
ηc,max of 86.70 cd Aꢀ1, EQEmax of 27.63% and ηp,max of 65.81 lm Wꢀ1. 2, the efficiency values are also as high as 84.42 cd Aꢀ1, 26.10% and
This device even shows lower efficiency rollꢀoff. For example, at the 55.22 lm Wꢀ1. These results suggest that four complexes are potential
practical brightness of 1000 cd mꢀ2, these efficiency values are 81.79 materials for efficient OLEDs.
Table 2. Key EL data of devices G1ꢀG4.
Devices
(8 wt%)
Von
Lmax
η
c
(cd Aꢀ1)c
at 1000 cd mꢀ2
79.97
η
p
(lm Wꢀ1)d
at 1000 cd mꢀ2
54.59
EQE(%)e
at 1000 cd mꢀ2
CIE
(V)a
(cd mꢀ2)b
(x,y)
Max
92.00
86.70
85.93
90.29
Max
67.53
65.81
69.19
Max
28.90
27.63
26.61
27.97
G1
G2
G3
G4
3.5
3.4
3.5
3.5
38155
35541
35927
36067
25.16
26.01
24.23
26.10
(0.27,0.51)
(0.21,0.57)
(0.23,0.62)
(0.23,0.62)
81.79
57.07
78.07
53.29
84.42
65.93
55.22
b
c
d
e
aTurnꢀon voltage, the driving voltage at 1 cd mꢀ2; Maximum luminance; Current efficiency; Power efficiency; External quantum efficiency calculated
within visible spectrum region.
(4ꢀ(Trifluoromethyl)phenyl)boronic acid (1.00 g, 5.26 mmol) and
2ꢀbromopyridine (0.75 g, 4.78 mmol) and Pd(PPh3)4 (0.28 g, 0.24
Experiments
General synthesis of the ancillary ligands
mmol) were added to a degassed mixture of toluene (25 mL), ethanol
(5 mL) and 2 M potassium carbonate aqueous solution (5 mL). The
mixture was refluxed for 24 h under nitrogen and extracted with
DCM (2 × 50 mL). The solvent was removed and the residue was
passed through a flash silica gel column with petroleum ether (PE)/
ethyl acetate(EA) (V/V = 8/1) as the eluent to give a white solid with
1ꢀ(Pyridinꢀ2ꢀyl)ethanone (1.26 g, 10.4 mmol) and esters (20.8 mmol,
2.0 equiv) were slowly added to a stirred suspension of sodium
hydride (0.50 g, 20.8 mmol) in dry THF (50 mL) at 0 0C. After
refluxing for 12 h, the mixture was quenched with 2N HCl to pH 5ꢀ6
and extracted with ethyl acetate. The organic phase was washed with
brine and concentrated to yield the crude 1, 3ꢀdione. Then hydrazine
monohydrate (104 mmol, 10 equiv) was added to the 1, 3ꢀdione in
50 ml of ethanol and refluxed for 24 h. Then the solution was
concentrated and the residue was dissolved in ethyl acetate. After
washing with brine and concentrated, the crude product was purified
through a flash silica gel column to give white solid.
1
a yield of 78%. H NMR (400 MHz, CDCl3) δ 8.75 ꢀ 8.73 (m, 1H),
8.13 (s, 1H), 8.11 (s, 1H), 7.84 ꢀ 7.79 (m, 1H), 7.79 ꢀ 7.76 (m, 1H),
7.74 (d, J = 8.2 Hz, 2H), 7.33 ꢀ 7.30 (m, 1H). MS (ESI): m/z 223
[M+].
General synthesis of Ir(III) complexes
Under N2 atmosphere, tfmppy (0.60 g, 2.70 mmol) and IrCl3.H2O
(0.32 g, 1.08 mmol) were added to a mixture of 2ꢀethoxyethanol (15
mL) and water (5 mL) and the solution was heated at 110 °C for 24
h. After cooling, the mixture was poured into distilled water (50 mL)
and the precipitate was collected and washed with water (2×10 mL).
The crude precipitate was dried in vacuum for 2 h to give chloroꢀ
bridged dimmer as yellow solid (0.62 g). Then, the chloroꢀbridged
dimmer with different ancillary ligands (2.07 mmol) and K2CO3
(0.95 g, 6.9 mmol) in 2ꢀethoxyethanol (15 mL) were refluxed for 24
h. After cooling, the mixture was poured into 50 mL distilled water.
It was extracted with DCM (2 × 50 mL) and the solvent was
removed, and the residue was purified through a flash silica gel
column with PE/ EA (V/V = 1/3) as the eluent to give light yellow
solid. The target Ir(III) complexes were further purified by vacuum
sublimation to give light yellow crystal.
mepzpy (yield: 60%) 1H NMR (400 MHz, DMSO) δ 12.72 (s,
1H), 8.55 (d, J = 4.8 Hz, 1H), 8.02 ꢀ 7.66 (m, 2H), 7.38 ꢀ 7.12 (m,
1H), 6.58 (s, 1H), 2.27 (s, 3H). MS (ESI): m/z 159 [M+].
cf3pzpy (yield: 62%) 1H NMR (400 MHz, DMSO) δ 14.34 (s,
1H), 8.68 ꢀ 8.66 (m, 1H), 8.00 ꢀ 7.91 (m, 2H), 7.47 ꢀ 7.41 (m, 1H),
7.38 (s, 1H). MS (ESI): m/z 213 [M+].
pypzpy (yield: 65%) 1H NMR (400 MHz, DMSO) δ 13.73 (s, 1H),
8.63 (d, J = 2.8 Hz, 2H), 7.99 (d, J = 7.0 Hz, 2H), 7.88 (s, 2H), 7.45
(s, 1H), 7.35 (s, 2H). MS (ESI): m/z 300 [M+].
phpzpy (yield: 62%) 1H NMR (400 MHz, DMSO) δ 13.55 (s, 1H),
8.63 (d, J = 4.5 Hz, 1H), 7.94 (s, 1H), 7.90 (d, J = 6.9 Hz, 1H), 7.86
(d, J = 7.6 Hz, 2H), 7.46 (t, J = 7.4 Hz, 2H), 7.36 (d, J = 6.5 Hz, 3H).
MS (ESI): m/z 299 [M+].
Synthesis of tfmppy
This journal is © The Royal Society of Chemistry 20xx
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DOI: 10.1039/C8TC01182F
ARTICLE
Journal Name
Ir-me (yield: 35%) 1H NMR (400 MHz, CDCl3) δ 7.94 (d, J = 8.1 There are no conflicts to declare.
Hz, 1H), 7.88 (d, J = 8.0 Hz, 1H), 7.77 ꢀ 7.74 (m, 1H), 7.74 ꢀ 7.71
Notes and references
(m, 1H), 7.71 ꢀ 7.69 (m, 2H), 7.68 (d, J = 3.7 Hz, 2H), 7.63 (d, J =
5.8 Hz, 1H), 7.55 (d, J = 5.5 Hz, 1H), 7.26 (s, 1H), 7.21 (dd, J = 8.1,
1.2 Hz, 1H), 7.17 (dd, J = 8.1, 0.9 Hz, 1H), 7.08 ꢀ 7.05 (m, 1H), 6.98 1. Y. Feng, X. Zhuang, D. Zhu, Y. Liu, Y. Wang and M. R. Bryce,
ꢀ 6.96 (m, 1H), 6.90 (dd, J = 9.8, 4.5 Hz, 1H), 6.49 (t, J = 7.2 Hz,
J. Mater. Chem. C, 2016, 4, 10246ꢀ10252.
3H), 2.33 (s, 3H). HRMS (m/z): calcd for C33H22F6IrN5[M+H]+ 2. D. P. Gong, T. B. Gao, D. K. Cao and M. D. Ward, Dalton
796.1487, found 796.1484.
Trans., 2016, 46, 275ꢀ286.
Ir-cf3 (yield: 40%). H NMR (400 MHz, CDCl3) δ 7.99 (d, J = 3. P. Li, Q.ꢀY. Zeng, H.ꢀZ. Sun, M. Akhtar, G.ꢀG. Shan, X.ꢀG.
1
7.9 Hz, 1H), 7.94 (t, J = 8.8 Hz, 2H), 7.87 (t, J = 8.2 Hz, 1H), 7.84ꢀ
7.79 (m, 2H), 7.77 (d, J = 7.1 Hz, 1H), 7.73 (d, J = 8.2 Hz, 2H), 7.65
Hou, F.ꢀS. Li and Z.ꢀM. Su, J. Mater. Chem. C, 2016, 4, 10464ꢀ
10470.
(d, J = 5.2 Hz, 1H), 7.53 ꢀ 7.47 (m, 1H), 7.25 (s, 1H), 7.20 (d, J = 8.1 4. Y. Wang, S. Wang, J. Ding, L. Wang, X. Jing and F. Wang,
Hz, 1H), 7.14 (t, J = 6.5 Hz, 3H), 6.99 (t, J = 6.2 Hz, 1H), 6.45 (s,
Chem. Commun., 2016, 53, 180ꢀ183.
1H), 6.40 (s, 1H). HRMS (m/z): calcd for C33H19F9IrN5[M +H]+ 5. Y. C. Zhu, L. Zhou, H. Y. Li, Q. L. Xu, M. Y. Teng, Y. X.
850.1204, found 850.1206.
Ir-py (yield: 38%). 1H NMR (400 MHz, CDCl3) δ 8.53 ꢀ 8.51 (m,
Zheng, J. L. Zuo, H. J. Zhang and X. Z. You, Adv. Mater., 2011,
23, 4041ꢀ4046.
1H), 7.96 (t, J = 8.5 Hz, 2H), 7.89 (d, J = 7.9 Hz, 1H), 7.81 (dd, J = 6. S. Lee, S. O. Kim, H. Shin, H. J. Yun, K. Yang, S. K. Kwon, J.
5.8, 0.8 Hz, 1H), 7.79 ꢀ 7.76 (m, 1H), 7.75 ꢀ 7.72 (m, 2H), 7.71 (s,
J. Kim and Y. H. Kim, J. Am. Chem. Soc., 2013, 135, 14321ꢀ
1H), 7.71 ꢀ 7.67 (m, 1H), 7.59 (dd, J = 7.2, 1.6 Hz, 1H), 7.57 ꢀ 7.51
14328.
(m, 1H), 7.41 (s, 1H), 7.26 (s, 2H), 7.23 (dd, J = 8.3, 1.3 Hz, 1H), 7. Q.ꢀD. Ou, L. Zhou, Y.ꢀQ. Li, S. Shen, J.ꢀD. Chen, C. Li, Q.ꢀK.
7.20 (dd, J = 8.2, 1.1 Hz, 1H), 7.05 ꢀ 7.03 (m, 1H), 7.03 ꢀ 7.00 (m,
1H), 6.98 ꢀ 6.96 (m, 1H), 6.93 ꢀ 6.91 (m, 1H), 6.56 (d, J = 0.8 Hz,
Wang, S.ꢀT. Lee and J.ꢀX. Tang, Adv. Funct. Mater., 2014, 24,
7249ꢀ7256.
1H), 6.53 (d,
C37H23F6IrN6[M + H]+ 859.1596, found 859.1591.
Ir-ph (yield: 36%). 1H NMR (400 MHz, CDCl3) δ 7.99 (d, J = 7.6 9. H. H. Kuo, Y. T. Chen, L. R. Devereux, C. C. Wu, M. A. Fox,
J
=
0.7 Hz, 1H). HRMS(m/z): calcd for 8. C.ꢀH. Yang, M. Mauro, F. Polo, S. Watanabe, I. Muenster, R.
Fröhlich and L. De Cola, Chem. Mater., 2012, 24, 3684ꢀ3695.
Hz, 2H), 7.97 ꢀ 7.85 (m, 4H), 7.75 (dd, J = 13.3, 8.1 Hz, 4H), 7.67 ꢀ
7.57 (m, 2H), 7.33 (t, J = 6.6 Hz, 2H), 7.26 (s, 2H), 7.23 (d, J = 7.0
C. Y. Kuei, Y. Chi and G. H. Lee, Adv. Mater., 2017, 29.
1702464ꢀ1702471.
Hz, 2H), 7.10 (s, 2H), 6.99 (dd, J = 8.3, 4.8 Hz, 2H), 6.50 (d, J = 8.2 10. C. Fan, Y. Li, C. Yang, H. Wu, J. Qin and Y. Cao, Chem. Mater.,
Hz, 2H). HRMS (m/z): calcd for C38H24F6IrN5[M + H]+ 858.1643,
found 858.1640.
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Conclusions
In summary, four efficient green emissive Ir(III) complexes (Ir-me,
Ir-cf3, Ir-py and Ir-ph) were obtained with 2ꢀ(4ꢀ
trifluoromethyl)phenylpyridine as main ligand and substituted 2ꢀ
(1Hꢀpyrazolꢀ5ꢀyl)pyridine derivatives as the ancillary ligands. Due
to the similar molecular structures, four materials exhibit little
different
photophysical,
thermal,
electrochemical
and
electroluminescent properties. Using these Ir(III) complexes as
dopants, the OLEDs showed high current efficiency with low
efficiency rollꢀoff. Especially, G1 device based on Ir-me complex
achieves the best device performances with the highest current
efficiency of 92 cd Aꢀ1 and the maximum EQE value of 28.90%.
Even at the brightness of 1000 cd mꢀ2, these efficiency values still
keep at 79.97 cd Aꢀ1 and 25.16%. This work proves an efficient
strategy to give the efficient green Ir(III) complexes by introducing a
variety of substituents into the ancillary ligands, which have
potential application in OLEDs.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (51773088) and the Natural Science
Foundation of Jiangsu Province (BY2016075ꢀ02).
Conflicts of interest
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C8TC01182F
Graphical abstract
100
80
60
40
20
G1
G2
G3
G4
0
1000
2000
Luminance
3000
4000
5000
(
cd m-2
)
Four efficient green iridium(III) complexes were applied as emitters in
the organic light-emitting diodes, which showing high current efficiency
of 92 cd A-1 and external quantum efficiency of 28.90% with low
efficiency roll-off.